Before assuming command of the Manhattan Project, Brigadier General Leslie R. Groves had directed the construction of the Pentagon, the huge office complex that housed the Department of Defense. Groves at first balked at the assignment to take charge of the Manhattan Project because he wanted a combat command. But when he realized that the atomic bomb could win the war for the United States, he enthusiastically brought his considerable energies and management skills to the project.

Within days of his appointment, Groves began in earnest. First, he needed a manufacturing site for separating uranium. As Niels Bohr had discovered in 1939, to make the bomb, a rare form of uranium — U-235 — was needed. This isotope of uranium is much more efficient in reacting with neutrons and producing a fission reaction than U-238, the common isotope found in...